Cognitive Radio (CR) is a promising solution to meet the increasing demands of wireless communications by letting the secondary users (SUs) opportunistically utilize the spectrum licensed to primary users (PUs). For this purpose, spectrum sensing is an inevitable step to identify the available spectrum holes for the SUs. Cooperative spectrum sensing (CSS) is a powerful method which outperforms conventional single-user sensing technology due to multiuser diversity. To apply CSS, the fusion rule in the fusion center (FC) plays an important rule in obtaining a satisfying sensing performance. In this paper, we shall investigate the optimal fusion rule for combining hard decisions from energy-based detectors so as to maximize the spectrum sensing efficiency. For a given false alarm probability requirement, we will determine the optimal threshold for the K/N fusion rule. It is shown that the K/N fusion rule outperforms the AND and OR fusion rules in terms of sensing efficiency.